© 2025 • Sales & Technical Support: (406) 587-4910 • email: sales@teamWavelength.com • web: www.teamWavelength.com

Case Study CS-LDTC16 Rev. A

Ultra-Sensitive CO-LITES

Detection at the Parts per

Quadrillion Level

June, 2025

Page 1

ABSTRACT

Researchers from Harbin Institute of Technology, China have developed a gas sensor for carbon

monoxide (CO) using light-induced thermoelastic spectroscopy (LITES). The ultra-sensitive design

enables CO detection in the parts per quadrillion (ppq) range using a multi-pass cell with a double helix

pattern generated using an artificial fish swarm algorithm as well as a polymer-modified round-head quartz

tuning fork with a low resonant frequency. The system achieves CO detection at concentrations as low as

23 parts per trillion (ppt) and 920.7 ppq by extending the integration time of the sensor. With a compact

design, high signal-to-noise ratio, and a long gas absorption path the CO-LITES sensor provides a new

standard in gas detection sensitivity for applications including semiconductor manufacturing, hydrogen

fuel cells, planetary exploration, and other fields needing highly sensitive trace gas detection.

SENSITIVE GAS DETECTION

The ability to detect trace gases in industrial, environmental,

energy, and scientific research applications can help avoid

major consequences that these low concentrations of gases

can cause. A highly sensitive technology can help advance

semiconductor manufacturing, energy innovation, and

interplanetary exploration. When producing semiconductor

chips, even trace impurities within the electron gases can

negatively impact the yield and reliability. When dealing

with energy innovation, trace carbon monoxide (CO) by-

products of fuel cells can degrade the performance of the

fuel cell or cause failure. Interplanetary exploration requires

sensitive sensors in the search for life and understanding

planetary atmospheres and the gases they hold.1 Whether

it's energy efficiency, manufacturing, or exploration, trace

gases in the parts per trillion (ppt) or even in the parts per

quadrillion (ppq) could have a significant impact on safety,

performance, health, or the environment.

For highly sensitive measurements and detection, laser

absorption spectroscopy (LAS) has gained traction among

many researchers. LAS uses lasers tuned to specific gas

absorption lines to detect a particular gas and to quantify

the concentration of the trace gas. Because of its rapid

response and high sensitivity, further developments have

produced quartz-enhanced photoacoustic spectroscopy

(QEPAS), advantageous due to its small size, low cost,

and high Q factor. Using LAS, or some form of LAS, can

provide ultra-high sensitivity gas detection for a variety of

applications in the manufacturing, energy, and exploration

fields.

PROBLEMS AND GOALS

Although QEPAS has significant advantages compared to

other non-laser absorption techniques, there is a notable

drawback of its design that can become problematic. In

the QEPAS design, a quartz tuning fork (QTF) must be

used to detect changes from the laser passing through the

gas environment, detecting thermal or acoustic signals.

When the QTF is submerged in acidic or corrosive gases,

the QTF's surface can become damaged, affecting the

sensitivity and properties of the QTF.

Another issue of the design of QEPAS is the short

absorption path restricting further advancement of detection

capabilities. As the Beer-Lambert Law dictates, a longer

path length of the light from the laser corresponds to a

stronger absorption of that light from the gas through which

it passes. If the gas absorption path is increased, the

detection capability of the system can be improved. This

is harder to achieve with the shorter path length design

of QEPAS, and the exposed QTF makes the technique

restrictive in its range of applications.

To solve these issues, light-induced thermoelastic

spectroscopy (LITES) can invert gas information from

thermoelectric signals from the QTF which can change

depending on the stimulation of the laser light and how

much is absorbed after passing through the sample gas.

This provides a non-contact method for sensitive gas

detection. However, the typical two-mirror multi-pass cell

(MPC) used with LITES to increase the effective optical

path length can be limited in precision. Three-mirror MPCs

can be used but may have complex optical structures to

fully unlock the potential for ultra-sensitive gas sensors.